(1) Field of the Invention
This invention relates to the casting of metals, particularly aluminum and aluminum alloys, by direct chill (DC) casting techniques. More particularly, the invention relates to the co-casting of metal layers by direct chill casting involving sequential solidification.
(2) Description of the Related Art
Metal ingots are commonly produced by direct chill casting of molten metals. This involves pouring a molten metal into a mold having cooled walls, an open upper end and (after start-up) an open lower end. The metal emerges from the lower end of the mold as a solid metal ingot that descends and elongates as the casting operation proceeds. In other cases, the casting takes place horizontally, but the procedure is essentially the same. Solidification of the ingot emerging from the mold is facilitated and ensured by directing streams of liquid coolant (normally water) onto the sides of the nascent ingot as it emerges from the mold. This is referred to as “secondary cooling” of the ingot (primary cooling is effected by the cooled mold walls). Such casting techniques are particularly suited for the casting of aluminum and aluminum alloys, but may be employed for other metals too.
Direct chill casting techniques of this kind are discussed extensively in U.S. Pat. No. 6,260,602 to Wagstaff, which relates exclusively to the casting of monolithic ingots, i.e. ingots made of the same metal throughout and cast as a single layer. Apparatus and methods for casting bi- or multi-layered structures (referred to as “composite ingots”) by sequential solidification techniques are disclosed in U.S. Patent Publication No. 2005/0011630 A1 to Anderson et al. Sequential solidification relates to the casting of bi- or multi-layers and involves the casting of a first layer (e.g. a layer intended as an inner layer or “core”) and then, subsequently but in the same casting operation, casting one or more layers of other metals (e.g. as outer or “cladding” layers) on the first layer once it has achieved a suitable degree of solidification.
U.S. Pat. No. 5,148,856 which issued to Mueller et al. on Sep. 22, 1992, discloses a casting mold provided with deflector means for deflecting the coolant streams in a variable direction depending on the local shrinkage conditions of the ingot being formed such that the coolant impinges on the ingot at a constant distance around the periphery of the ingot. The deflector means is preferably a movable baffle.
While these techniques are effective, difficulties may be encountered when attempting to employ the sequential solidification technique with certain combinations of alloys, particularly those having similar and, especially, overlapping freezing ranges on cooling from the molten state (i.e. overlapping ranges between the solidus and liquidus temperatures of the respective alloys). In particular, when such metals are sequentially cast, it is sometimes found that the cladding layer may not bond as securely to the core layer as would be desired, or the interface between the cladding and core layers may rupture or collapse during casting due to high contraction forces generated in the various layers.
There is therefore a need for improved casting equipment and techniques when co-casting metals of these kinds.